1. Field of the Invention
The present invention relates to a mobile communication terminal used in a cellular mobile communication network system and a cell search method for the mobile communication terminal.
2. Description of the Related Art
In a cellular mobile communication network system, a plurality of base stations are distributed in a service area. A radio zone called a cell is formed by each base station. Radio connection between a base station and a mobile communication terminal is executed in each cell. In a system of this type, when a mobile communication terminal is powered on, synchronization is established between the mobile communication terminal and the nearest base station. After establishment of synchronization, the mobile communication terminal is set in a standby state. When, in this standby state, the base station notifies the mobile communication terminal of an incoming call, or an operation for an outgoing call is executed by the mobile communication terminal, negotiation for establishing a communication link is executed between the base station and the mobile communication terminal. When the communication link is established, the mobile communication terminal is set in a communication state.
In the cellular mobile communication network system, handoff is performed as the mobile communication terminal moves from a cell to another. Handoff is processing for changing the base station as the synchronization establishment destination of the mobile communication terminal. Preparing for handoff, the mobile communication terminal in the standby state or communication state receives a pilot signal transmitted from a base station (to be referred to as an active base station hereinafter) for which synchronization is established and measures the reception level while receiving pilot signals transmitted from neighboring base stations located in the neighborhood and measures the reception levels. It is determined on the basis of the measurement result whether handoff is necessary. When it is determined that handoff is necessary, handoff processing is executed.
In this handoff control method, if the reception quality of a signal from a neighboring base station is higher than that of a signal from the active base station, handoff processing is executed even when the reception quality of the signal from the active base station is so high that no handoff is required. For this reason, unnecessary handoff frequently occurs depending on the transmission path environment around the mobile communication terminal. If both the signal reception quality from the active base station and those from the neighboring base stations are poor, and the difference between them is small, handoff is repeatedly executed. In addition, synchronization establishment fails more frequently due to errors in selecting the handoff destination. Under these circumstances, the power consumption of the mobile communication terminal increases. This shortens the service life of the battery.
Conventionally, processing for measuring the reception quality of a signal from each neighboring base station in accordance with the transmission path environment around a mobile communication terminal, i.e., a technique for increasing/decreasing the frequency of cell search for neighboring base stations has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 2001-28778 (see pp. 1, 9, and 10 and FIG. 4)). According to this technique, when the transmission path environment around a mobile communication terminal is satisfactory, the number of times of cell search for neighboring base stations is small. Hence, the power consumption of the mobile communication terminal decreases.
However, although the number of times of cell search for the neighboring base stations is decreased, this technique has no regard for processing for measuring the reception quality of a signal from the active base station. Hence, the power consumption of the mobile communication terminal is still large.